Abstract:
The structure and oxidation state of self-assembled nanostructures formed by oxidation of aniline with ammonium persulfate in the presence of alanine have been investigated by solid-state 13C and 15N NMR, FTIR, GPC, elemental, UV−vis, and SEM methods. These techniques have been applied to samples obtained 1 and 20 h after the beginning of the reaction in their doped as-synthesized form or after reduction with hydrazine or dedoping with NH4OH or LiOH. Peaks at 66 and 72 ppm and a peak at 241 ppm in the 15N NMR spectra are shifted downfield by about 1−6 ppm and upfield by about 80 ppm, respectively, relative to the positions expected for the amine and imine N atoms in polyaniline. This indicates a difference in molecular structure between the nanotube material and standard polyaniline. This may be attributed to the presence of either strong H-bonding or a fundamentally different bonding connectivity in the nanotube material. Despite the significant difference in morphology of the samples after 1 and 20 h as shown by SEM, the 13C and 15N spectra show resonances at very similar chemical shifts for the two reaction times. The differences in line width and intensity of the peaks in these spectra are attributed to an increased number of positive charges and to longer polymer chains for the longer reaction time. The previously postulated presence of phenazine-like units in the molecular structure within the nanotubes cannot be excluded due to possible overlapping of resonances from amine and tertiary nitrogens in the 15N NMR spectra. The carbon and nitrogen NMR spectra show that the alanine is not incorporated into the aniline oxidation products. Chemical structures whose presence is consistent with the data obtained from solid-state NMR and the other methods applied in this work are proposed.
